Reorganize files

Reorganizes the documents to be more in line with my planned teaching for fall
2013; instead of having separate 'point estimation' and 'inference' documents,
I have 'finite sample' and 'asymptotics'
Draft introductory random variable material

Draft of an intro to the probability theory section; it tries to motivate
probabilty measures from densities.  Formally, I guess it's trying to
informally define probabilities from Radon-Nikodym derivatives w/rt
Lebesgue measure.